Plainweave 1.2.1

Error-legibility (functional-honesty) fixes surfaced by clean-room dogfooding: an error must
say what it knows, and a hint that doesn't point at the real fix misdirects. Additive and
backward-compatible — the weft.plainweave.error.v1 envelope and the ErrorCode vocabulary
are unchanged.

Fixed

• Version-conflict errors now disclose the actual version. A CONFLICT on a requirement
  version or a draft revision previously reported only "expected version does not match current version" with empty details, forcing callers to probe for the real value. Both optimistic-
  concurrency guards now name the expected and current values in the message, in structured
  details ({expected_version, current_version} / {expected_draft_revision, current_draft_revision}), and in a hint that names the recovery
  (e.g. Retry with --expected-version 0.).
• Cause-specific error hints; dropped the misleading blanket hint. _error no longer stamps
  "Refresh local Plainweave state and retry." on every error — it misdirected, e.g. a missing
  --actor VALIDATION error, down a dead path. Each error now carries a cause-appropriate hint
  from a per-ErrorCode map (VALIDATION/NOT_FOUND never claim staleness; CONFLICT legitimately
  suggests a refetch), with precise hints for the missing-actor, verification-method /
  evidence-status, and trace-relation cases, plus an honest INTERNAL hint on the MCP
  preflight-severity guard.